In silver halide photography, a latent image is formed in the silver halide emulsion layer of a photographic element by image-wise exposing the emulsion layer to radiation. The exposure step is followed by development, fixing, washing and drying steps. Radiographic elements, that is, photographic elements used in radiography, are commonly processed in automatic processing machines in which the element is guided from one processing station to another. In these machines loss of activity of the processing solution is counteracted by continual or periodic replenishment of processing solutions. Processing, in these automatic machines is generally conducted at elevated temperatures, in excess of 30.degree. C., to reduce processing time. Emulsions used for radiographic elements usually have a low ratio of binder, such as gelatin, to silver halide. A hardening developer is used with these elements to avoid softening and excessive swelling of the emulsions. Hardening developers must provide rapid development since as emulsion layers harden, permeability decreases.
Sequestering agents are used in photographic developers to counteract the effect of soluble salts or trace metal impurities that may be present. Such impurities may originate in the developer itself or may be introduced from the environment during use of the developer solution. Common impurities are calcium, iron, and copper ions. Calcium can precipitate in the developer resulting in particulate contamination. Iron and copper can catalyze the oxidation of hydroquinone or the like, resulting in a degradation of developer stability. These effects are particularly undesirable in developers used in radiography.
Radiographic developers generally have pH values in the range of about 9-11. Development is accelerated at higher pH levels, however, so is developer breakdown associated with oxidation.
Sequestering agents typically function by forming stable complexes with metal ion impurities; thus reducing the concentration of free metal ion impurities to acceptable levels. These complexes are classified in Photographic Processing Chemistry, L.F.A. Mason, Focal Press, London, (1975) pp. 55-67, by structure into three main groups: complex phosphates, hydroxyacids, and nitrogenous carboxylic acids.
Complex phosphates are good sequestering agents for calcium and magnesium ions at high pH, but as a class have limited stability in alkaline solution.
Hydroxyacids have limited use in developer solutions since they are poor calcium sequestering agents and have poor sequestering power at high pH.
Nitrogenous carboxylic acids do not have the same limitations as complex phosphates and hydroxyacids. Although some nitrogenous carboxylic acid sequestering agents have the shortcomings of forming stable complexes with silver ions and having limited thermal stability, many nitrogenous carboxylic acids have been widely used in developers (see U.S. Pat. No. 3,201,246). This class of sequestering agents includes triacetic acid compounds (see, for example, U.S. Pat. No. 4,040,833), tetraacetic acid derivatives (see, for example, U.S. Pat. No. 3,994,729), and other more complex derivatives (see, for example, U.S. Pat. No. 4,672,025). Specific examples of nitrogenous carboxylic acid sequestering agents include: iminodiacetic acid, methyliminodiacetic acid, hydroxyethyliminoacetic acid, carboxyethyliminodiacetic acid, nitrilotriacetic acid, ethylendiaminetetraacetic acid, glycol ether diaminetetraacetic acid, 1,3-diamino-2-propanoltetraacetic acid, diethylenetriaminepentaacetic acid, and triethylenetetraminehexaacetic acid.
Hydrolyzed polymaleic anhydride is marketed commercially for use as a calcium carbonate inhibitor in photoprocessing solutions, especially chromogenic color developer solutions. Hydrolyzed polymaleic anhydride is available from the Additives Division of Ciba-Geigy Corporation, Hawthorne, N.Y., under the trademark Irgaform 3000. This material is also available under the trademeark Belclene 200 from the same source. Hydrolyzed polymaleic anhydride is a liquid having a pH of about 1, which is neutralized by alkali with the evolution of heat.
It is desirable to provide an alkaline hardening developer which includes a sequestrant that has good stability at the pH of the developer, is a good sequestering agent for calcium and other metals such as iron, and, at least under optimal conditions, does not cause adverse sensitometric effects upon radiographic elements developed in an automatic processor. It is also desirable to provide a development method utilizing that developer.